Radiation Safety, Protection and Dose Control
Framework
Radiation protection rests on justification, optimisation (ALARA) and dose limitation. Absorbed dose is measured in gray (J/kg); equivalent and effective dose in sievert, obtained by applying radiation weighting (1 for X and gamma rays) and tissue weighting factors. Typical ICRP limits: 20 mSv per year averaged over five years for occupational exposure, with no single year above 50 mSv; 1 mSv per year for the public; 20 mSv per year for the lens of the eye; 500 mSv per year for extremities. Dose constraints sit below the limits.
The three controls
Time, distance and shielding. Dose rate from a point source falls with the square of distance, so doubling distance quarters the dose rate. Shielding calculations use half-value and tenth-value layers. Collimation reduces both scatter and the irradiated volume. Controlled and supervised areas are defined by dose rate and annual dose; local rules, a radiation protection supervisor and personal dosimetry (TLD, OSL or film badge) provide the administrative layer. Sources must be secured, accounted for and moved under a written procedure, and emergency plans must cover stuck or disconnected sources.
Traps
Applying the inverse-square law to a large or extended source; forgetting that scatter, not the direct beam, often dominates behind a shield; confusing mSv with µSv; mixing up public and occupational limits; and ignoring that source output also falls with isotope half-life.
Worked example
A source gives 400 µSv/h at 1 m. At 4 m the rate is 400/16 = 25 µSv/h. To reach 7.5 µSv/h, distance must be √(400/7.5) ≈ 7.3 m, or additional shielding must be added.